US2553449A - Process for obtaining by galvanic means a coating for protecting magnesium from corrosion - Google Patents

Description

Patented May 15, 1951 U NIT ED STAT PATENT LC E. 1

PROCESS FOR OBTAINING. BY" GALVANIC MEANS A. COATING FOR PROTECTING" MAGNESIUM FROM CORROSION HerbertManfred Freud, Nanterre, France No Drawing. Application April'18,'1946, 'Se'r'ial' No. .663, 24'8. In Franc'eJuIy-ZB, 1945 6 Claims. (Cl. .204--56 1, In my copending application (on which Patent No. 2,463,483 issued March 1,1949) Serial No.

314,349, filed January 17,1940, I described a procr.

ess for obtaining by. galvanic. means a coating.

for protecting magnesium from corrosion, such coating being produced by a galvanic process.

The preferred solution, which was used in that process contained the .bi-valent ion Mn++ associated with an anion which does. not attack the metal chemically, such as 01720:".

In the process. set forth in Ser..,.No.,3,l4,3,4.9, several manganese salts suchIas the nitrates, the chlorides, the sulphates, were stated not to be. capable of producing a good protective layer on magnesium, because they produce an attack on the metal.

The improved process which is the object of the present invention is related to this same technique of action by galvanic means as opposed to. action by chemical means, and herein, as in Ser. 314,349, no electric current from outside is supplied.

One of the greatadvantages of protecting magnesium by galvanic means under the conditions of the present case (in addition to the improved resistance to corrosion) is that it is possible, in certainconditions, and in .particular by. the process which is the object of application Serial ,No.. 314,349, to provide an excellent. protective coating without producing aconsiderable change of dimensions, when treating calibrated parts.

Before starting the description of the process which is the object of the present invention and in order to make more apparent the importance of; this fundamental distinction which exists between the action by chemical means and action by galvanic means, some indications will first of all be given concerning the phenomena which characterise the action by chemical means in the reactions relating to the treatmentof magnesium.

The chemical attack of magnesium is generally manifested by one of the. two,fo'1lowingphenomena:v

1.. A progressive dissolution of a surface-portionof the metal, without leaving. any chemically produced. covering occurs, and the amount of the metal so dissolved can be evaluated .by the loss in weight of the metal during its immersion in the solution.

Thus, a plate of 1 *dm' (total surface :area, on

both sides) of an-alloy of magnesium contain-.

ing'1.8% of manganese was immersed in :a solutien containing 0l-6.-% rot iMnt ion .and;1-.0%=.oi; CIOs obtained zby dissolving .100:- :.gr.. of .CI'QB .in. 999120. of Water-tand .iadding..;thereto 19 gr; :of

MnCOs. Thisv soLutiomhasuaipI-I less .than 1.1 It is difficu-lt to accurately determineizhegafl pf: this solution by the scientific methodsiknownyat present, :owing. tolthe presence @iia large propor tion of 01103, thelex'idizingpowerioiwhich is;.:cone; siderable, butv the pH; isnlessi thazn .l,. measu-red any; the. glass e1ectro'de.l-.,Said z p-late weighed 9.125 gr; before it wasintroduced into thezbath, and 8.944; gr. afterremainingxtherein enethour; The lossiofi. weight therefore. .0384; -gr... The; decrease- "in; thicknessof the plate was more. than 2/100.

(i. .e. somewhat over 9.02 mun);

:2... Dissolution pf a qaortion .of the :.meta-l-- and; the. :simu1taneous...coaifing.; of; the metal with .wa layer of which :thexcelour varies from yellow to; mordo-r (brownish-.xmimson), brown or; black. This. layer. .ismore orzless; adherent to themagnesium and-according tozitsthickness, is :of -great-.-. er;-;or;- lesser :weight-avhichqmay to. some extent; counterbalance the thickness and wei ht of metal lostby solution.

Thuspa ,plate of 1 dm -itotalzsuraface :area, :on: both sides) of ansallny ,ccntaining.1:.8% or ganese was immersed for an hour in a solution containing 10% of 'Naz'croiand 8% of KHSO4 (pH=5.7). Said plate 'initiallyiweighed 9.087 gr., and 9.039 gr. after-remaining in the bath for one hour. During: that. treatment it becamev covered with a ,black filmdotted with-"yellow spots. Theapparent loss of weight was therefore only. Q 0 048 gr.

But, when this .blaek-filmwasremoved, by suc--- cessive immersions-in ,hot: solutions oi soda and of. sodium hyposulphite :(NazSzOs. which is oommonly called hypoii, the-plate weighed -onl y- 8.954 gr. The rear .lossaof weight in the. metal i itself iis thereforeO .1 33 g-r era decrease. of thick. ness of the metal-sheetnfzabout.0.0115 mm;

7 In the absence oiionsnther:thanMnH and; CrtOm-- or. ICIOPCT, the chemical attack takes: place only if the pH-of; the bath 'is :very low, i. .e. much-v -below 2:5. contradistinction, a the presence of-ions-such ars 0.4, theichemical attack takes placeat a pH ofrnearly'fiyorat any =pI-I lower than this if the bath aconsiderable;

concentration oflsuchzionsl.

My research 1 work has enabled me-ate. discover the-conditions under; which with maths-in which:

. the acid-ion is composed :principally .i0I.;.CIi2O11'0r;-:

CrO4 the galvanic action takes place without there being a chemical attack on the magnesium, and on the other hand to discover that, as an exception to the general rule, mentioned in application Ser. No. 314,349, and the accuracy of which has been practically confirmed by practice, that the presence of sulphates, chlorides or nitrates produced an attack on the metal, the chemical action did not occur if the acid ion concentration is maintained within certain limits hereinafter defined.

Consequently, the object of the present invention is a process for protecting, by galvanic action, metal objects, by means of a bath mainly containing as anion one of the CrzOv or CrO4 ions and which consists in operating with certain conditions of pH and of concentration which are defined hereinafter:

1. According to the invention, when the operation is effected in the absence of ions other than Mn++ and CrzOq-- or CrO4 the pH should be maintained, in order that the galvanic action may be effected in satisfactory conditions, between 2.5 and 3.5 for a fresh bath. It is advisable in practice to keep the pH at about 3.

I have found that at a pH below 2.5 and at a pH much above 3.5, the treatment is too slow for the application of the process to be satisfactory industrially.

2. The operation may be effected in the presence of very small quantities of an active mineral acid, which will be obtained by the addition of ions such as SO4--, N03", PO4-, either in the form of acids, or in the form of salts. In particular S04" gives good results whether it is introduced in the form of sulphuric acid or of sulphates, or again in the form of sulphonated organic acids. Under these conditions (i. c. with a very small amount of H2804 added), an electrolyte having a pH up to 3.95 may be satisfactory for use, (see table below).

For this addition of these acid ions not to produce any chemical attack, the bath must be such that the potential at the magnesium electrode formed by the object to be treated is less than 1.1 volts.

The concentration of such active mineral acid ions should be less than 0.5% and in particular it should be less than the figures:

The pH should be higher than 2.5. The upper limit of 3.5 is mentioned as being inadvisable to exceed owing to the slowness of the galvanic action, when the Mn++ and CIv or CrOrions alone are present in the electrolyte. But this limit may be considerably exceeded in the presence of sulphuric, nitric or phosphoric acids or their salts since the presence of small quantities of the acid radicals of these acids increases the oxidizing power of the bath.

The table given hereinafter, which gives, for a series of proportions of constituents, the value of "the pH and that of the potential at the magnesium electrode, relatively to the hydrogen electrode, illustrates the fact that according to the proportions, either a chemical attack occurs (which is very undesirable), or, on the contrary, only a galvanic action.

In practice, for these measurements, in view of the present difficulty in operating with hydrogen electrodes, I have used as a reference electrode a calomel electrode with saturated potasan electromotive force of 1.9 volts.

Item Bath composition (per liter) E 10 grs. CrOa and 5.8 grs.

MnCO

3. Item a plus 0.1 cc. HzSO4.- Item a plus 0.3 cc. H2SO4 grs. CrOa and 58 grs.

M11003. Item 11 plus 0.1 cc. HzSO4. l0 grs. CrOa and 6 grs.

nCO

Nam on NRO HUIN chemical attack.

The figures in the column marked E, in the above table show the computed potential of the magnesium electrode relatively to the hydrogen electrode.

If, instead of the S04 ion, N03 and P04 ions 'were used, a similar table would be obtained.

In the latter cases also chemical attack occurs when the potential at the magnesium electrode relative to the hydrogen electrode, is greater than 1.1 volts.

It should be noted that the small quantities of N03 ions carried along as a result of the usual depolarising operations of carbons by means of nitric acid, followed by athorough washing, do not substantially alter the limits given in either of the above mentioned cases. However, it is possible, on the contrary, to limit the washing so as to allow sufiicient N03 ions to remain for the acidity to be maintained within the above mentioned limits.

By way of examples, given for the sole purpose of facilitating the comprehension of the present description, it is possible to proceed as follows:

Example I A solution is prepared by dissolving in 1 litre of water, 10 gr. of CrOs, and 5.8 gr. of manganese carbonate are then added, and then 0.100. of sulphuric acid. The pH of this bath is 3.2.

A plate of an area of 1 dm (total surface area, on both sides) of magnesium alloy containing the circuit, a current of 70 milliamps flows, at a potential of 0.9 volts. 7

After a suitable period of treatment, (e. g. one hour), this piece is withdrawn, washed and dried; it is covered with a dark grey coating, and weighs 9.001 gr. The coating formed therefore weighs 0.026 gr.

Example II Into a solution prepared from 1 litre of water,

10 gr. of C103 and 6 gr. of MnCOs, there has been carried (from the .depolarisation of the.

By closing carbons Tinflhlilrlfhacldllflfil grlSofii HNOa The bath. ,accordingly contains...about 0.05%...I-IhIQ3.

,The "pH of -this bath is 3.3.

. -.A plate of an area'of 1 dm lttotal surfacelarea,

mersion in the bath and 9.016 gr. after an immersion of "one-hour. There is therefore no chemical attack whatever.

.;.:-If carbon counterelectrode/is; placed opposite ithis magnesium alloy plate, a-'cell iswobtained r having an electromotiveforce .(in open'circuit) of 1.78 volts. By closing the circuit, a current of 55 milliamps'fiows at 0.85 volt.

Aftera .suitable period Jof. treatment I hour, for example), the piece is withdrawn; washed'and .nrlried ;It is covered. with albrown film:and:w.eighs The coating formed therefore-weighs Example "III To a solution prepared by dissolving 100 gr.

of chromic acid in 1 litre of Water, are added 66::

gr. of MnCOs, 1.2 gr. of MnSO4 corresponding to 0.75 gr. of S04 ion. The pH of the bath is 3.7. A plate made of magnesium alloy containing 2.7% of aluminium, 0.3% of manganese and 1% of zinc and having an area of 1.2 dm (total, for both sides of plate) is immersed therein.

The potential of the Mg relatively to the hydrogen electrode is 0.98 volts. When the circuit is open the electromotive force between the 1 g and a counter-electrode of carbon is 1.88 volts. By closing the circuit a current of 60 milliamps flows at 0.90 volts. After a period of treatment of hour, the piece is withdrawn, washed and dried.

The durations of the galvanic action mentioned hereinbefore may vary according to the thickness of the protective layer to be obtained. If the duration is too short, the layer is insufiiciently thick; if it is too long, the layer is too thick but a decrease of adherence is observed and it may have a tendency to scale.

The operations are carried out at temperatures of from 0 to 50 C. and preferably at room temperature.

I claim:

1. A process of providing a protective coating upon a metallic article composed substantially entirely of magnesium and magnesium base alloy, by galvanic action, which comprises placing such an article to be protected, as anode in an acid electrolyte having a pH value above 2.5, but not substantially above 3.95, and such electrolyte being incapable of chemically attacking metallic magnesium, and said electrolyte consisting essentially of water containing a soluble manganese compound of chromic acid, said electrolyte also containing a small amount of at least one substance containing the anion of a mineral acid which mineral acid would in higher concentration be capable of chemically attacking magnesium, such acid being selected from the group consisting of sulphuric acid, nitric acid and phosphoric acid, the amount of such mineral acid ion being not more than 0.1% of sulphate ions, not more than 0.5% of nitrate ions, not more than 0.2% of phosphate ions, and placing a cathode in said electrolyte, but out of physical contact with said anode, such cathode being an electrically conductive material more noble than,

magnesium, electrically connecting said anode and said cathode outside said bath, the electrode "i6 apotential 10f: rthe: v"magnesium;:electro de :inasaid iielectrolyte relatively; to: .the hydrogen; electrode .:.:-b'eing.=: not over .1.Lavolts,rtcontinuing' the=fiowr0f Jsaidocurrentx'until a. protective .film of substan- .cof SOrt radical, the potential atsaidmagnesium eano'dew in :saidielectrolyte: relatively .to theJhy'dro- -.'gen-;electrode :being:1ower..-than .1 .1 1 volts, and :the pI-Irof the electrolyte. being higher than 2.5;1but not above. 3.95,..and rsaid .electrolyte being isfree otmotherusubstances.whichwvould affect the pH and the electrode potential of said magnesium anode.

-:;.3.-.- A;-.process-, as T covered in, 1 claim ,1, ,in which the cathode is carbon.

4. A process of providing a protective coating upon a metallic article composed substantially entirely of magnesium and magnesium base alloy, by galvanic action, which comprises placing such an article to be protected, as anode in an acid electrolyte having a pH value above 2.5, but not substantially above 3.95, and such electrolyte being incapable of chemically attacking metallic magnesium, and said electrolyte consisting essentially of water containing a soluble manganese compound of chromic acid, said electrolyte also containing a small amount but not over 0.1% of a substance containing S04 ions, and placing a cathode in said electrolyte, but out of physical contact with said anode, such cathode being an electrically conductive material more noble than magnesium, electrically connecting said anode and said cathode outside said bath, the electrode potential of the magnesium electrode in said electrolyte relatively to the hydrogen electrode being not over 1.1 volts, continuing the flow of said current until a protective film of substantial thickness has been formed on said anode.

5. A process of providing a protective coating upon a metallic article composed substantially entirely of magnesium and magnesium base alloy, by galvanic action, which comprises placing such an article to be protected, as anode in an acid electrolyte having a pH value above 2.5, but not substantially above 3.95, and such electrolyte being incapable of chemically attacking metallic magnesium, and said electrolyte consisting essentially of water containing a soluble manganese compound of chromic acid, said electrolyte also containing a small amount, but not over 0.2% of a compound containing P04. ions, and placing a cathode in said electrolyte, but out of physical contact with said anode, such cathode being an electrically conductive material more noble than magnesium, electrically connecting said anode and said cathode outside said bath, the electrode potential of the magnesium electrode in said electrolyte relatively to the hydrogen electrode being not over 1.1 bolts, continuing the flow of said current until a protective film of substantial thickness has been formed on said anode.

6. A process of providing a protective coating upon a metallic article composed substantially entirely 'of magnesium and magnesium base alloy,

substantially above 3.95, and such electrolyte being incapable of chemically attacking metallic magnesium, and said electrolyte consisting essentially of water containing a soluble manganese compoundof chromic acid, said electrolyte also containing a small amount but not over 0.5% of a compound containing N03 ions, and placing a .cathode in said electrolyte, .but out of physical contact with said anode, such cathode being an electrically conductive material more noble than magnesium, electrically connecting said anode' and said cathode outside said bath, the electrode potential of the magnesium electrode in said electrolyte relatively to the hydrogen electrode being not over 1.1 volts, continuing the flow of said current until a protective film of substantial thickness has been formed on said anode.

HERBERT MANFRED FREUD.

REFERENCES CITED The following references are of record inthe file of this patent:

' UNITED STATES PATENTS Number Name Date 1,838,273 McBride Dec. 29, 1931 2,052,962 Booe Sept. 1,1936 2,206,028 Buzzard July 2, 1940 2,224,528 Sutton etal Dec. 10, 1940 2,356,575 Frasch Aug. 22, 1944 2,463,483 Frasch Mar. 1, 1949 FOREIGN PATENTS Number Country Date 7 515,648 Great Britain Dec. 11, 1939 702,266 France Apr. 3,1931 471,053 Germany Jan. 17, 1929,

OTHER REFERENCES AlienProperty Custodian publication 297,191

20 v 87 through 91.

Transactions of The Electrochemical Society," vol. 90 (1946), pages 503 through 507.

Claims (1)

1. A PROCESS OF PROVIDING A PROTECTIVE COATING UPON A METALLIC ARTICLE COMPOSED SUBSTANTIALLY ENTIRELY OF MAGNESIUM AND MAGNESIUM BASE ALLOY BY GALVANIC ACTION WHICH COMPRISES PLACING SUCH AN ARTICLE TO BE PROTECTED, AS ANODE IN AN ACID ELECTROLYTE HAVING A PH VALUE ABOVE 2.5, BUT NOT SUBSTANTIALLY ABOVE 3.95, AND SUCH ELECTROLYTE BEING INCAPABLE OF CHEMICALLY ATTACKING METALLIC MAGNESIUM, AND SAID ELECTROLYTE CONSISTING ESSENTIALLY OF WATER CONTAINING A SOLUBLE MANGANESE COMPOUND OF CHROMIC ACID, SAID ELECTROLYTE ALSO CONTAINING A SMALL AMOUNT OF AT LEAST ONE SUBSTANCE CONTAINING THE ANION OF A MINERAL ACID WHICH MINERAL ACID WOULD IN HIGHER CONCENTRATION BE CAPABLE OF CHEMICALLY ATTACKING MAGNESIUM, SUCH ACID BEING SELECTED FROM THE GROUP CONSISTING OF SULPHURIC ACID, NITRIC ACID AND PHOSPHORIC ACID, THE AMOUNT OF SUCH MINERAL ACID ION BEING NOT MORE THAN 0.1% OF SULPHATE IONS, NOT MORE THAN 0.5% OF NITRATE IONS, NOT MORE THAN 0.2% OF PHOSPHATE IONS, AND PLACING A CATHODE IN SAID ELECTROLYTE, BUT OUT OF PHYSICAL CONTACT WITH SAID ANODE, SUCH CATHODE BEING AN ELECTRICALLY CONDUCTIVE MATERIAL MORE NOBLE THAN MAGNESIUM, ELECTRICALLY CONNECTING SAID ANODE AND SAID CATHODE OUTSIDE SAID BATH, THE ELECTRODE POTENTIAL OF THE MAGNESIUM ELECTRODE IN SAID ELECTROLYTE RELATIVELY TO THE HYDROGEN ELECTRODE BEING NOT OVER 1.1 VOLTS, CONTINUING THE FLOW OF SAID CURRENT UNTIL A PROTECTIVE FILM OF SUBSTANTIAL THICKNESS HAS BEEN FORMED ON SAID ANODE.